1. Field of the Invention
This invention relates to a pan coating apparatus for applying pharmaceutical coating layer, sugar coating or film coating onto a powdery material, a granular material such as a tablet and a granular foodstuff, and the like.
2. Related Art Statement
In order to apply a sugar coating or a film coating onto a powdery material, a granular material such as pharmaceutical tablets, granular food stuffs, or the like, there has been used a pan coating apparatus, which is otherwise called a granulating-coating apparatus or a tablet coating apparatus. The fundamental structure of the pan coating type apparatus is disclosed in U.S. Pat. No. 3,834,347. The apparatus of No. 3,834,347 has been improved variously and is most commonly used at present.
The pan coating apparatus of No. 3,834,347 is shown in FIG. 6(A), and has a rotary drum 1 called a coating pan which is rotated about a substantially horizontal rotary axis. Shaft portions 2 and 3 are provided at opposite end portions in the axial direction of the rotary drum 1. The rotary drum 1 is made rotatable by bearings, not shown, for supporting these shaft portions 2 and 3.
An openable lid 4 is opened, and a powdery or granular material such as tablets is supplied into the rotary drum 1. Coating liquid is sprayed through a nozzle 6 onto a layer 5 of the powdery or granular material accumulated at a bottom portion of the rotary drum 1.
In order to dry the coating liquid applied onto the powdery or granular material, the largest diameter portion of the rotary drum 1 is formed of porous plates having a multitude of air holes 7, and a multitude of air duets 8 communicated with the interior of the rotary drum 1 through the air holes 7 are provided on the outer periphery of the rotary drum 1 along the axial direction, respectively. Air for drying, which is introduced into the rotary drum 1 through an air supply duet 9, flows Through the accumulated layer 5 of the powdery or granular material, thereafter, is guided into the air ducts 8 positioned at the lower side of the rotary drum 1 in accordance with the rotating position of the rotary drum 1, and is discharged into an air exhaust duet 11 through a distributor 10.
Pan coating apparatuses have the above-described fundamental structure, and are of various types depending upon air introducing and exhausting systems.
FIG. 6(B) is a view showing another pan coating apparatus similar to the above-described apparatus, and corresponds to a cross-section of FIG. 6(A). In the air introducing system in this case, air is supplied or exhausted through an air duct 8a which is positioned at the upper side of the rotary drum 1 in accordance with the rotating position of the drum 1 and substantially opposed to the accumulated layer 5, and air is exhausted or supplied through an air duct 8b positioned at the lower side of the rotary drum 1 and under the accumulated layer 5 due to the rotation. The communication of the air supply or exhaust ducts and the upper and lower air ducts 8a and 8b is achieved by a distributor, not shown. The pan coating apparatus having this structure is described in Japanese Patent Laid-Open No. 60-500894, which corresponds to U.S. Pat. No. 4,586,457 and German patent No. 3310031.
FIG. 6(C) is a view showing a schematic construction of a pan coating apparatus described in Japanese Patent Laid-Open No. 2-207833, which corresponds to U.S. Pat. No. 5,050,528 and German Pat. No. 69001827. In the air introducing system of this apparatus, an inner duct 9a for supplying air is projected into the rotary drum 1 and the air is blown out of a cut-away portion 9b of the inner duct 9a into the rotary drum 1, and the air blow-out direction is made variable.
In the above-described conventional apparatuses, air can be supplied through the air exhaust duct 11 and exhausted through the air supply duct 9, whereby the direction of air flow can be inverted.
FIG. 6(D) is a view showing a schematic construction of an apparatus described in Japanese Patent Laid-Open No. 5-309253, in which an opening 12 communicated with the air supply duct 9 is formed in a central portion of one end of the rotary drum 1 and a hemispherical or truncated cone-shaped airflow control plate 13 is provided in the rotary drum 1 in a manner to cover this opening 12. The central portion of the airflow control plate 13 is blind and through-holes are formed in the other portion.
However, the following disadvantages are presented in the conventional apparatuses, in which the above-described various air introducing systems are used.
Firstly, in the apparatus shown in FIG. 6(A), air is caused to flow from the side of a hollow shaft portion 2 toward below a shaft portion 3 in the rotary drum 1. When, particularly, a sectional area of the air supply duct 9 for supplying air into the rotary drum 1 is small, the flow rate of the introduced air becomes high, whereby liquid drops of the sprayed coating liquid are caused to flow away, so that a satisfactory spray pattern cannot be obtained, and the coating liquid adheres ununiformly to the powdery or granular material.
When the sectional area of the air supply duct 9 is increased, the flow rate is decreased and the turbulence of the spray pattern is reduced. In that case, however, the air flow course leans toward the left side of the rotary drum 1 in the drawing, whereby the air flow through the accumulated layer 5 at the right side becomes unsatisfactory, so that the drying efficiency is decreased.
The apparatus shown in FIG. 6(B) is constructed such that air is introduced from the air duct 8a positioned upwardly in the rotary drum 1, so that such an advantage is offered that air flows moderately from above to below over the whole rotary drum 1, so that the spray pattern is not turbulent. However, when an air duct is rotated to the upper position 8a, it is used for introducing air, and when the same air duct is rotated to the lower position 8b, the air duct is used for the exhaust of air. Thus, if dust of dried particles of the sprayed coating liquid and of worn granular material intrudes into the air duct during the exhaust of air, then, when the air duct is rotated to the upper position 8a, the dust is blown back into the rotary drum 1 and adheres to the surfaces of the powdery or granular material, thus making the finish of the coating unsatisfactory.
In this apparatus, when a type of a powdery or granular material to be coated is changed, dust generated by the processed powdery or granular material and by dried particles of a coating liquid remains in the air ducts and adheres to a new powdery or granular material. Therefore, particularly when a pharmaceutical powdery or granular material is to be processed, a serious problem is caused from the viewpoint of GMP (Good Manufacturing Practice). Therefore, when a type of a powdery or granular material is to be changed, the air ducts should be carefully cleaned, thus taking a long time for the cleaning.
In the apparatus shown in FIG. 6(C), the flow rate of air at the air blow-out portion is high and the turbulence of the spray pattern is caused although there is difference depending upon the position of the inner duct 9a which is projected into the rotary drum 1, the dimensions of the air blow-out opening 9b, and the direction of air blow-out. Furthermore, even if the blow-out opening is positioned downwardly, air flow in the inner duct 9a becomes substantially of high rate, whereby air is blown out toward the left side of the rotary drum 1 in the drawing, and there may occur adherence of the coating liquid to the inner surface of the rotary drum 1, or a so-called dusting phenomenon, in which the liquid drops of the coating liquid are dried to form particles before they reach the powdery or granular material. If the pipe diameter of the inner duct 9a is increased, this disadvantage may be relaxed to some degree. However, from the construction of the apparatus, in which the inner duct 9a is projected into the rotary drum 1, the pipe diameter cannot be increased to the extent where the disadvantage can be entirely obviated.
When a direction of air flow is inverted in the above-described three types of apparatuses, air flows against spraying of the coating liquid, whereby the spray pattern becomes extremely turbulent, and further, such a disadvantage is presented that dusting tends to occur.
Particularly, when sugar is coated on a powdery or granular material by use of the apparatus shown in FIG. 6(C), a part of the powdery or granular material is raised by the rotation of the rotary drum 1, thereafter, drops onto the top surface of the inner duct 9a and adheres thereto or is accumulated thereat. Thereafter, when the adhered powdery or granular material falls and contaminates the finished granules which has been coated, a variation in weight between the granules is increased and defective products may be produced in some case.
In the apparatus shown in FIG. 6(D), air is blown out toward the whole outer periphery of the rotary drum 1, so that air flow in the rotary drum 1 becomes turbulent and the spray pattern becomes unsatisfactory. Moreover, the airflow control plate 13 is projected into the rotary drum 1, whereby, when sugar coating is applied, the raised granular material adheres to the airflow control plate 13, so that the disadvantage similar to the case of FIG. 6(C), where the inner duct is used, is presented. Further, in the case that the airflow control plate 13 is projected into the rotary drum 1 in a manner to cover the opening 12, when the interior of the apparatus including the air supply duct 9 is cleaned, cleaning liquid remains in the airflow control plate 13 and the cleaning liquid intrudes into the air supply duct 9, whereby cleaning works become troublesome. Further, it is undesirable from the viewpoints of GMP (Good Manufacturing Practice) and validation.